Unvented hot water storage systems save space in the loft when compared to traditional systems and can supply large volumes of hot water on demand. One of the key benefits is that the hot water supply is delivered at mains pressure right across the house. However because of the possible dangers associated with water stored in a sealed vessel under pressure and at high temperature, it is important to have the system serviced once a year. This service will amongst other things test the functionality of the safely valves that operate in the event of a fault.